I don't think they even used repeats like this back then, so the expected behavior would probably be based upon individual opinion. The Dutch lady who found this expected the dots to line up with the bars, which make more sense to me than to have them offset.

From my experience (and as the original implementor of the Mensurstrich feature), I don't think there is an "expected behaviour". Mensurstrich is a purely typographic device used only in modern editions of early music; its purpose is to add hints for measure bars when the original source was lacking them (as it was customary until approx. first half of XVII c.), without breaking too much the "free flow" of "un-barred" music printing.

@mike320 : repeats WERE used "back then", under several forms, occasionally needing some interpretation (and raising very scholarly arguments ;) ); for instance attached is a potential source of the example in the OP (from Dowland's First Booke of Songes originally published in 1597, here from a 1613 reprint), which shows the barred Cantus and the unbarred Tenor, both with a final repeat indication.

Being present in the source, the repeats should not be rendered with Mensurstrich, but with regular on-staff measure bars, which would avoid any problem with the dots. From my experience, I would assume this to be by far the most common case.

Sure, one case remains uncovered: when a repeat is an editorial addition, but I am far from sure that repeat dots between the staves would look any better than those shown in the OP; even the repeat bars do not look very nice between the staves and contradict the Mensurstrich purpose, which is to have the measure bars as incospicuous as possible. Personally I would engrave the (editorially added) repeat normally within the staves, adding about it a note in the apparatus.

In summary, it seems to me that:

1) it is quite rare a case;
2) the proposed solution is not significantly better than the current one

and this does not seems to justify the added complexity of managing it.

What I meant was there were not bar lines between staffs as in current Mensurstrich notation. Mensurstrich did not exist when the music was originally written, so there was no issue raised as to where to put dots and lines in these situations. It seems more logical to me (an apparently the OP) to have the dots line up with the bars rather than the current situation of having them offset.

@mike320 : sure, you are right, Mensurstriche are a modern compromise, to add bar lines -- with the minimum visual impact -- where there was none, at the same time indicating that they are added.

One can argue that this does not really apply to repeat signs, but only to bar lines. If the repeat signs are original, they are not "added" and should not go between staves, but within the staves (together with their (double) bar lines), where they originally were.

If they are an editorial addition, then I have no obvious solution, but I would like to see a real case, before spending time and increasing the code complexity without a compelling reason (the example proposed by the OP seems an especially contrived case; certainly, nothing in the source of Come again suggests that repeats should be added there).

OP confirmed that the repeat bars are an editorial addition. She acknowledged the version with the lute that you posted as well as the version in 4 voices. OP claimed that the repeats are the way all modern versions notate the song. This seems to be a bit of a hybrid between modern and ancient notation, which is what this notation is anyway. Nonetheless OP and I both agree the notation should make sense. If you do not want to fix it, I understand since there are many things in MuseScore that are in more need of being fixed than this rarely used feature. I just request that this be left open as a bug report in case someone at some point decides they would like to fix it.

While mike320 conclusion is probably reasonable, I am still convinced that there is no "real" bug: while I did not foresee this specific development while implementing the feature (as I never expected someone would use it in this non-standard way), Mensurstrich is not actually intended to be applied and should not be applied to repeats; it is intended as a philological notation and the feature is currently as it is essentially by design.

In other words, the repeats should remain within the staves, regardless of the position of other bar lines. Does the OP knows that each bar line can have its own span independently of the other, isn't it?
If an arranger feels to add repeat indications, he is obviously free to do it, but an arrangement is hardly the context where to use philological notations...

Generalising and as an aside: what about arbitrary alterations to the span of a repeat? If you alter it to encompass only part of a staff, would you expect the dots to move up or down to remain centred on the line segment? To me it would look very odd.

You made a good point about each bar line being able to be adjusted independently, and I will point that out to the OP. As far as the dots moving with the repeat bar line. I expect the dots to remain centered on the bar line at all times, no matter how much of the staff it encompasses. Unlike single lines, I've never seen repeat lines less than the distance of a staff. I know that when the repeat bar line spans multiple staffs the dots are centered on each staff, but I consider the lines between the staffs to be connectors rather than part of the bar line. In Mensurstrich the "connectors" are the bar lines. This is a matter of perspective supported by the fact that if you shorten a bar line spanning a grand staff to single staff, you will end up with two independent bar lines.